Such a dual-mode filter typically includes a circular resonator, such as a ring resonator, having a circular periphery, which is formed on a substrate made of a dielectric material, and a pair of input/output ports connected to the circular resonator through a capacitance for external circuit coupling, which is formed on the substrate at a symmetrical position with respect to a symmetry plane passing through the center of the circular shape of the circular resonator. Perturbation stubs are formed on the substrate to extend radially along the symmetry plane of the circular resonator at the diametrically opposite sides of the circular resonator. The pair of input/output ports is disposed outwardly in diametral direction with respect to the circular resonator in a manner to define a right angle therebetween.
In the paper of Lei Zhu, et al, "A Joint Field/Circuit Design Model of Microstrip Ring Dual-Mode Filter: Theory and Experiments", 1997 Asia Pacific Microwave Conference, pp. 865-868, several forms of such type of dual-mode filters are described.
As typically shown in FIG. 2 of the aforementioned paper, in the conventionally proposed dual-mode filters, the length of the stub pole should be changed in order to optionally determine an attenuation pole frequency of the filter. However, when the length of the perturbation stub was changed, there causes a problem that a coupling condition of the filter would be swerved from a critical coupling condition so that a passbandwidth of the filter would be simultaneously changed. Thus, in such conventional dual-mode filters, it has been highly difficult to optionally set an attenuation pole frequency with maintaining a constant passbandwidth of the filter.